1. Field of the Invention
This invention relates to high frequency-weldable articles, such as sheets, fibers, and yarns, and particularly to high frequency-weldable articles that include at least one high frequency-weldable part made from a modified polyethylene terephthalate material. This invention also relates to a method for connecting the high frequency-weldable articles through high frequency welding techniques.
2. Description of the Related Art
Some articles of thermoplastic materials that possess a loss factor property, such as plastic sheets that are made from polyethylene and polypropylene and that are used in food packaging, and filters of air conditioners that are made from polyvinyl chloride or polyurethane, can be laminated quickly and evenly through high frequency welding techniques without using solder material. However, since the use of these halogen-containing materials, such as polyvinyl chloride, will be restricted due to environmental concerns, the relevant industries have attempted to develop high frequency-weldable packaging sheets and fiber filters that are made from halogen-free thermoplastic materials.
Polyethylene terephthalate (PET) is a halogen-free and inexpensive saturated ester that is commercially available and that has a wide range of applications. However, since pure PET has no loss factor property, the articles, such as the plastic sheets, fibers or fiber articles, made therefrom are not suitable for use with high frequency welding techniques.
In recent years, modification of PET and manufacture of PET articles have been focal points in the art of polymers. U.S. Pat. No. 5,585,177 discloses a laminate including a triple layered thermoplastic resin laminated on one or both sides of a metal sheet. The inner layer, which is proximate to the metal sheet, and the outer layer of the triple layered thermoplastic resin are made from modified polyesters. Preferably, the inner and outer layers may be made from isophthalic acid (IPA)-modified PET. Alternatively, the inner layer may be made from a blended polyester resin that is prepared by blending a pure PET (melting point: 256° C.) and a butylene glycol-modified PET (melting point: 252° C.) in equal amounts. However, inclusion of the metal substrate results in a higher production cost and difficulty when recycling the laminate.
U.S. Pat. No. 6,551,699 discloses a calendered sheet made from a calendering composition comprising a modified polyester and a fatty ester. The modified polyester is 1,4-cyclohexanedimethanol (CHDM)-modified PET. The crystallization half time of PET may be prolonged through modification of 1,4-cyclohexane-dimethanol so as to favor the calendering operation. However, the modified polyester can only be dried at a temperature lower than 70° C. When pellets of the modified polyester are processed to form desired articles, a lengthy time for drying or pre-crystallizing the pellets of the modified polyester is required, which results in an increase in the manufacturing costs.
U.S. Pat. No. 5,804,025 discloses a shaped structure made from a modified polyester through high frequency welding techniques. The shaped structure is in the form of a high frequency-weldable staple fiber web that consists of a first fiber and a second fiber. The first fiber is made from PET modified with 33 mol % IPA.
Conventionally, the modified PET is normally produced by addition of bivalent acids, particularly IPA, during polymerization of PET. When IPA is not added to PET in a sufficient amount, such as less than 33 mol %, the modified PET thus made, in spite of being suitable for use in the production of high frequency-weldable thermoplastic sheets, cannot be spun into high frequency-weldable yarns. In other words, the application of the modified PET thus formed is limited.
As the demand for packaging sheets and fiber products is relatively large, there remains a need in the art to improve the high frequency-weldable sheet or fiber product, such as yarn, that is made from a halogen-free, inexpensive modified PET.